This invention concerns a tool fabricated of polycrystalline cubic boron nitride (PCBN or CBN) for machining ferrous metals.
Cubic boron nitride is a superhard material in the same class as diamond although not quite as hard as diamond. Diamond can be used for machining many materials because of its great hardness. It is not generally considered suitable for high speed machining of ferrous metals since iron catalyzes decomposition of diamond at elevated temperatures and there is, in effect, a chemical decomposition or erosion of the diamond. CBN is not as susceptible to thermal decomposition by iron and is, therefore, suitable for some machining of ferrous metals where diamond cannot be used.
A PCBN cutting tool may be formed by high temperature, high pressure processing on a cemented tungsten carbide substrate. In such a tool, the polycrystalline CBN is infiltrated with cobalt. This invention concerns a different type of CBN cutting tool where the body of PCBN is "solid" or "unsupported" and substantially free of infiltrated cobalt. In other words, a blank of PCBN is made by high temperature, high pressure processing without a carbide substrate. An unsupported PCBN cutting tool is formed from the starting powders and usually infiltrated or mixed with aluminum and/or silicon. The resulting blank is cut and ground to a desired tool shape. The tool is clamped in a tool holder for a lathe for example. Such a CBN cutting tool is described in the context of machining cast iron, but it may also be used for other ferrous or non-ferrous metals, or for non-metallic work pieces.
A CBN cutting tool is still subject to erosion or wear and requires chemical and thermal resistance for optimizing the cutting rate on a workpiece and the lifetime of the tool.
A few manufacturers have PCBN cutting tools in the marketplace. One such cutting tool marketed by Megadiamond is made from a mixture of grain sizes of CBN particles, a small amount of diamond particles and is infiltrated with an aluminum--silicon eutectic alloy as a sintering catalyst.
It is common to define the composition of the tool after high temperature, high pressure processing by the ingredients used to make the tool. This is because the processed tool may end up with CBN with an apparent particle size that is different from the particle size of the initial ingredients, but very hard to measure. Similarly, an infiltrant containing aluminum and silicon ends up as a complex mixture of aluminum nitride, silicon nitride, silicon carbide, aluminum oxide and/or silicon oxide, which are quite difficult to distinguish from each other. These compounds may collectively be referred to simply as a second phase.
The exemplary Megadiamond PCBN composition comprises about 80% (by weight) of 22 to 36 micron CBN, about 10% 12 to 22 micron CBN, about 7% CBN with smaller particle sizes and about 3% fine diamond crystals. The composition is infiltrated with a eutectic aluminum--silicon alloy (about 88% by weight aluminum, 12% silicon).
It may be noted that the particle size ranges are so-called particle size "cuts" as specified by CBN suppliers. The actual particle size in a specific cut tends to be somewhat smaller than the ends of the ranges stated for the cut, and the particle size distribution tends to be skewed toward the smaller particle sizes. Thus, for example, a 12 to 22 micron cut from one CBN vendor has actual particle sizes between about 10 and 17 microns and an average particle size of about 13 microns (average particle size is 50% by volume or mass). Furthermore, the cuts are defined such that the particle sizes are the 90% values. In other words, at least 90% of the particles are larger than the lower limit and at least 90% are smaller than the upper limit. Larger particles are not common. "Fines" (small particles) are often seen in cuts with larger particle sizes.
It is recognized by those skilled in working with these small particle size materials that particle size is not an exact science and involves some degree of approximation when defining the particle size. It is also recognized that the original starting material particle size can be roughly estimated from the particle size seen upon microscopic examination of a finished product.
The PCBN cutting tools involved in practice of this invention approach 100% CBN (e.g. 95% CBN, 5% TiCN as feed material). There are other compositions employed for other machining tasks having lower proportions of CBN and additional ingredients. Such compositions are shown, for example, in U.S. Pat. Nos. 4,647,546; 4,650,776; 5,271,749; 5,326,380; 5,639,285 and 5,697,994.
Both General Electric Company and DeBeers have solid or unsupported, approximately 100% PCBN cutting tools in the marketplace. A General Electric material known as BZN7000S appears to have a CBN particle size implying primarily an 8-12 micron cut. A material available from DeBeers' known as Amberite appears to employ a similar particle size cut, 8 to 12 microns. A PCBN material marketed by Seco as SECO 300 is similar, but has a slightly larger CBN particle size. Both of these materials include aluminum nitride and in addition the General Electric material appears to include silicon in the infiltrant.
Showa Denko has commercially available solid or unsupported PCBN cutting tools identified as KS-10 and KS-25. Examination of these materials indicates that KS-10 has approximately 80% CBN and 20% TiN. KS-25 appears to have 60% CBN and 40% TiN. Both include AlN. The average grain size of the CBN in both products is no more than about five microns.
As is often desirable, improved performance in service is a goal of development efforts. Thus, it is desirable to have a PCBN cutting tool which lasts longer and/or has a higher production rate.